Improvement of tightening uniformity of group threaded connections during aircraft units assembly

Аuthors

Solov'ev V. L.

Omsk State Agrarian University, Institutskaya St., 2, Omsk, 640008, Russia

e-mail: vladlen_solovev@bk.ru

Abstract

The improvement of methods (ways) and means of implementation and control of preload force remains one of the main directions of development of research on improvement of threaded connections reliability up till now.The research is focused on the process of assembly (tightening) of group threaded connections of aircraft units.
The paper is devoted to the problem of low accuracy of the control of the preload force according to the value of the torque moment during the assembly of group threaded connections of the repaired aircraft units.
The research aims at improving the tightening uniformity of the group threaded connections by increasing the accuracy of the control of the preload force according to the values of torque moments.
The accuracy of control of the preload force according to the value of the torque moment is largely determined by the magnitude of the friction forces in the threaded connection, which is the main flaw of such control method.
In actual practice the calculated values of friction coefficients can vary within a rather wide range from 0.05 up to 0.5, while the geometrical dimensions of a thread can deviate from the standards. These are the factors that result in low accuracy of such control method.
It was determined by an experimental approach that the condition of а threaded connection influences the relationship between the torque moment and the preload force. The experiments showed that after the threaded connection was exposed to corrosive action the previously calculated torque moment did not provide the required preload force even when a lubricant was used.
At present there are many ways and means of threaded connections tightening. However, the problem of providing the uniformity of distribution of the preload force in group threaded connections of aircraft units remains unsolved. This leads to numerous failures and decreases the reliability of the engineering.
It is possible to provide a more uniform tightening of group threaded connections by applying a more accurate method of preload force control by evaluating the relation between the unscrewing and screw-in torque moments.
The method consists in calculation of the exact torque moment needed for obtaining the required preload force for each threaded connection within the group. The relation of the unscrewing and screw-in torque moments, which is estimated for each particular threaded connection, makes it possible to take into account the actual condition of every threaded connection (i.e. indirectly estimate the values of the friction coefficients). At that it is necessary to take into account that the condition of the threaded connection may change during the term of unit operation.
The technological approach, which is described in this paper, allows to determine correctly the value of the relation between the unscrewing and screw-in torque moments for a particular threaded connection.
The paper contains useful information for scientists, who work on the development of threaded connection assembly technologies.

Keywords:

threaded connection, torque, preload force, friction coefficient, torque wrench

References

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